fix(lotw): stop the heartbeat when the report starts arriving

Deferred, it stopped at the end of the function — so 'still waiting for LoTW to
build the report' kept printing between the megabyte lines, saying the opposite
of what was happening.

internal/extsvc/lotw.go

@@ -12,6 +12,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"strings"
+	"sync"
 	"syscall"
 	"time"
 )
@@ -162,7 +163,11 @@ func DownloadLoTWConfirmations(ctx context.Context, client *http.Client, cfg Ser
 			}
 		}
 	}()
-	defer close(beat)
+	// Stopped where the WAIT ends, not where the function does: deferred, the
+	// heartbeat went on counting between the megabyte lines and read as if the
+	// report were still being built while it was already arriving.
+	stopBeat := sync.OnceFunc(func() { close(beat) })
+	defer stopBeat()
 
 	var resp *http.Response
 	for attempt := 1; ; attempt++ {
@@ -192,6 +197,7 @@ func DownloadLoTWConfirmations(ctx context.Context, client *http.Client, cfg Ser
 		return "", fmt.Errorf("lotw: request failed: %w", err)
 	}
 	defer resp.Body.Close()
+	stopBeat()
 	say(note, fmt.Sprintf("LoTW answered (HTTP %d) — receiving…", resp.StatusCode))
 	body, err := readWithProgress(ctx, resp.Body, note)
 	if err != nil {